Spiropyran groups

Photoresponsive polymers can be obtained by introducing photochromic units, such as azobenzene or spiropyran groups, into the macromolecules of polymeric compounds. As described in Chapter 1 of this book, photochromic compounds can exist in two different states, such as two isomeric structures that can be inter-converted by means of a light stimulus, and the relative concentrations of which depend on the wavelength of the incident light. For instance, in azobenzene compounds, photochromism is due to trans-cis photoisomerization around the N=N double bond, while in spiropyran compounds photochromism involves interconversion between the neutral spiro form and the zwitterionic merocyanine form (Figure 1). 

L. V. Natarajan, T. J. Bunning, and S. Y. Kim, Photochromic liquid crystalline cyclic siloxanes containing spiropyran groups, Macromolecules, 27, 7248-7253 (1994). 

I. Cabrera, V. Krongauz, and H. Ringsdorf, Photo- and thermochromic liquid crystal polymers with spiropyran groups, Mol. Cryst. Liq. Cryst. 755, 221-230 (1988). 

Ohya, Y., Okuyama, Y., Fukunaga, A., and Ouchi, T. (1998) Photo-sensitive lipid membrane perturbation by a single chain lipid having terminal spiropyran group, Supramol. Sci., 5, 21-29. 

Light-induced transformations from one structure to another have been studied with many modified polypeptides [13, 14] bearing pendant photochromic groups such as azobenzene or spiropyran groups. Typical examples are the modified poly(L-glutamic acids) PGA-1 and PGA-2 presented in Chart 5.5. 

The coil helix transition proceeds rapidly within seconds, whereas the back reaction requires several hours for full conversion. Notably, in this case, the photochromic behavior of the spiropyran groups is opposite to that observed in other solvents (see example (b) in Table 5-1). The reverse photochromism is due to the high polarity of hexafluoro-2-propanol, which stabilizes the charged mero-cyanine form better than the uncharged spiropyran form. 

These processes are called photochromic if considerable color changes occur during the photoisomerization. Poly(methacrylates) with certain spiropyran groups become colored on irradiation with light and subsequently decolorize slowly in the dark ... 

It was reported that siloxanes containing up to 20 % spiropyran groups are liquid crystalline. At higher compositions they are amorphous. With increased spiropyran content, the selective reflection band characteristic of cholesteric mesophases shifts to lower wavelength ... 

Another typical photoresponsive material for preparation of switchable surfaces is the spiropyran-merocyanine system. The spiropyran isomerizes to zwitterionic merocyanine conformation by UV exposure, and the reverse reaction can be triggered by irradiation with visible light as well as azobenzene. The changes in hydrophilic/hydrophobic properties through the isomerization of spiropyran groups also enable the control of cell adhesion/ detachment. Edahiro et al. reported photoresponsive cell culture substrates grafted... 

Many examples are known such as the Fries rearrangement in aromatic polyesters or in some poly(methyl methacrylates) with poly(spiropyran) groups. 

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